Permanent paper and method of making same



Patented Dec. 27, 1949 Q UNITED STATES PATENT OFFICE PERMANENT PAPER AND METHOD OF MAKING SAME No Drawing. Application September 28, 1948, Serial No. 51,653

Claims. 1

This invention relates to paper and similar products composed of felted cellulosic fibers having improved permanence and stability on storage and heating. The invention includes the improved paper and other felted cellulosic material and methods of producing the same.

It is a well-known fact that articles composed of or containing felted or molded cellulosic fibers such as paper, board, wet-molded articles and the like lose much of their resiliency with the passage of time. It is also known that this development of brittleness is accelerated by heating; in fact, hea ing is a standard test method to determine the expected normal aging of paper products. The present invention has as a principal object the incorporation into paper of a class of materials which will reduce the development of brittleness in paper and paper articles, and thus increase their efiective life.

It is also a known fact that paper and other felted fibrous articles containing acidic materials and normally having a pH of about 4 to 6 will deteriorate faster upon storage or heating than those which are completely neutral. For this reason the aging characteristics, for example, of ordinary writing paper sized with rosin size and alum, are not ordinarily as good as those of unsized or untreated papers. It is a further advantage of the stabilizing agents of the present invention that they will retard the embrittlement and discoloration of paper containing small amounts of residual acid or of acidic materials.

Our invention is based on the discovery that ammonium and alkali metal salts of dicyanimide are effective stabilizing agents to retard the deterioration of paper and other felted cellulosic products when incorporated therein in relatively small amounts, usually on the order of about 0.3% to about 10% by weight. We have found that the addition of relatively small amounts of these compounds to paper will prevent loss in tensile strength and in tear and fold resistance when the treated paper is subsequently aged and heated. Such improvements are achieved without simultaneously afiecting the color and sizing properties of the paper. Inasmuch as the paper stabilizing agents of the above class are all substantially colorless and neutral in character, they do not alter the appearance or utility of the paper for its intended purpose.

In practicing the process of our invention the ammonium or alkali metal salts of dicyanimide may be added to the paper at any desired point during or after it is formed on a paper-making machine. Since all of the stabilizing agents are water-soluble they can be incorporated into the paper very simply by impregnation from water solution, and this can be done in conjunction with sizing agents and the like during the finishing of the paper if desired. Thus, for example, water solutions of our stabilizing agents may be sprayed on the felted paper sheet, or may be applied as a tub size. When a closed white water system is used, they may be added to the paper stock suspension prior to formation of the sheet, and may be used along with other beater-sizing agents such as rosin size and alum, starch, glue, fillers, colors, resins and the like materials.

As is noted above, the minimum quantity of the stabilizing agents that will produce the desired result in paper are on the order of about 0.3%,based on the dry weight of the cellulose. For most purposes, satisfactory results are obtained when about 0.5% to about 5% of the stabilizing agent is used. Amounts of more than 10% usual- 1y produce no further advantages, and therefore are not recommended.

The ammonium and alkali metal salts of dicyanimide, any of which may be used in practicing the invention, are all soluble in water but are normally solid and colorless and therefore do not change the appearance or ink-receiving properties of the paper. These dicyanimide salts are prepared by reacting an alkaline alkali metal or ammonium compound with a heavy metal salt of cyanamid. For example, calcium cyanamid may be reacted with ammonium chloride, ammonium sulfate or the like and the ammonium radical replaces the heavy metal and forms the dicyanimide salt.

The invention will be further illustrated by the following specific examples. It should be understood, however, that although these examples may describe in detail certain specific embodiments of the invention, they are given primarily for purposes of illustration and the invention in its broader aspects is not limited thereto.

EXAM'PLEI The unaged and aged paper, together following table. In the headings of this table 4. Paper containing about 0.3 percentto 10 per- MD means machine direction," CD means cross cent by weight of ammonium dicyanimide. direction," and Refl. means reflectance measured 5. A method of improving the stability of paper at 450 millimicrons on a G. E. recording spectroagainst aging which comprises impregnating said photometer. paperwith an aqueous solution of a member se- Unaqed paper M. I. '1. ma Stabilizer Per Cent 33* Red.

MD on None (Control) 5. 4 87 '33 72 Sodium Dleyanimide 1.8 5. 6 76 24 72 Ammonium Dicyanimide 1. 6 i5. 4 77 26 71 1 Means the percent retention of stabilizer. based on the weight of the cellulose.

1 Means the percent retention of stabilizer, based on the weight of the cellulose.

EXAMPLE 2 A safety-check base paper having a relatively low pH was impregnated, dried. and tested as in Example 1, with the following results:

lected from the group consisting of ammonium and alkali metal salts of dicyanimide and subsequently drying the paper to retain therein about 0.3 percent to percent of said compound.

Unayed paper M. I. '1. Fold Stabilizer Per Cent E3 Refl.

MD OD None (Control) 4.0 Sodium Dlcysnimide. 1 4 6. 2 30 37 65 Poiasium Dicyanimido 1 5 6.3 31 40 66 1 Means the per cent retention of stabilizer, based on the weight 0! the cellulose.

Aged paper M. I. '1. Percent Fold shoot 7 Fold Retention Btebflizer Per Cent pH Refl.

MD CD MD OD None (Control) 3 8 2 4 60 5.7 10 Sodium Dicyanimide 1.4 6 0 21 25 50 67.6

Potaaium Dicyanimide 1.5 6 0 21 28 67 67.7 70

1 Means the per cent retention of stabilizer, based on the weight of the cellulose.

What we claim is:

1. Paper containing about 0.3 percent to 10 percent by weight of a member selected from the roup consisting of ammonium and alkali metal REFERENCES CITED salts of dicyanimide.

2. A product according to claim 1 in which the a; gig g zg are of record m the paper is a normally acid paper having a pH of UNITED STATES PATENTS about 4 to 6. i

Name Date JOHN BTUDENY. JOHN D. POLLARD. CHESTER G. LANDES.

3. Paper containing about 0.3 percent to 10 a Number percent by weight 01' sodium dicyanimide. 455 Nagy at a] 7 1948 

